[S390] KEYS: Enable the compat keyctl wrapper on s390x

Enable the compat keyctl wrapper on s390x so that 32-bit s390 userspace can
call the keyctl() syscall.

There's an s390x assembly wrapper that truncates all the register values to
32-bits and this then calls compat_sys_keyctl() - but the latter only exists if
CONFIG_KEYS_COMPAT is enabled, and the s390 Kconfig doesn't enable it.

Without this patch, 32-bit calls to the keyctl() syscall are given an ENOSYS
error:

	[root@devel4 ~]# keyctl show
	Session Keyring
	-3: key inaccessible (Function not implemented)
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: dan@danny.cz
Cc: Carsten Otte <cotte@de.ibm.com>
Reviewed-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-s390@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -227,6 +227,9 @@ config COMPAT
 config SYSVIPC_COMPAT
 	def_bool y if COMPAT && SYSVIPC
 
+config KEYS_COMPAT
+	def_bool y if COMPAT && KEYS
+
 config AUDIT_ARCH
 	def_bool y